1. Field of the Invention
The present invention relates to a method of manufacturing, in a semiconductor device manufacturing process which is not capable of preventing a carbon contamination such as a process which is conducted within a clean room using a hopper filter, a semiconductor device providing a clean semiconductor interface with the removal of that contamination. In particular, the present invention relates to a method of manufacturing a thin-film semiconductor device providing a clean semiconductor interface and a gate insulating film low in carbon density by removing a carbon contaminator on the interface between an active layer and the gate insulating film, as well as by removing carbon impurities in the gate insulating film which is formed using an organic silane source in a method of manufacturing a thin-film transistor (TFT) using a thin-film semiconductor such as a liquid-crystal display field.
2. Description of the Related Art
Up to now, in the method of manufacturing the semiconductor device, various contamination removing manners have been established for the removal of the contaminator on its surface as well as for the prevention of contamination as a problem. For the removal of heavy metal, there have been remarkably widely known methods in which heavy metal is removed by adding hydrochloric acid to hydrogen peroxide solution, and so on. Also, for the removal of a physical absorber, there have been well used a cleaning method using the cavitations of supersonic waves, a cleaning method using a brush, and so on. Moreover, even in the liquid-crystal display field where a large number of thin-film transistors are formed on an insulating substrate, using normal tetraethyl silicate chemical formula Si(OC.sub.2 H.sub.5).sub.4 (so-called TEOS) as a source gas, a so-called stepping of a thin-film transistor wiring and so on are reduced utilizing the excellence of the step coating property of the gas. Further, in the liquid-crystal display field using a process which is not at a high temperature used in a silicon wafer process but at 600.degree. C. or lower, TEOS has been used for a gate insulating film or an under layer in addition to an inter-layer insulating film. In the thin-film transistor (so-called TFT) which is applied to the liquid-crystal display or the like, an under film formed on an insulating substrate such as a glass substrate, a gate insulating film, an interlayer insulating film and so on are formed through the heat CVD method, the plasma CVD method or the like with normal tetraethyl silicate as a source gas. However, this suffers from a problem on the oxide film characteristic because a large amount of carbon remains.
For the removal of organic substances such as carbon that adheres to the surface, there have been well known a cleaning method using solvent where sulfuric acid is added to hydrogen peroxide solution, a dry ashing method using oxygen plasma, and so on. However, from the research of the inventors, it has been found that the removal of carbon is under more complicated circumstances. The cause of the mixture of carbon contaminations is that a photo-resist used for forming an intended pattern in a photolithography process is of a photo sensitive organic that causes a carbon contamination. Also, in the method of manufacturing the semiconductor device, the thin-film process is now essential, and a vacuum device for the thin-film process is also essential. However, there exists a vacuum pump in the vacuum device for making vacuum, yet using oil. This causes carbon contamination. In addition, the contamination is caused by a vapor pressure generated from teflon (PFA), polypropylene (PP), polyvinylidene fluoride (PVDF), ethylene trifluoride covalent resin (ECTFE), ethylene tetrafluoride covalent resin (ETFE), polyethylene (PE) tetrafluoride, or the like, which is used as a substrate carrier, a floor material within a clear room, a wall material, a filter, and so on.
A conventional method is that a dry ashing is conducted after a photolithography process, and a solvent in which sulfuric acid is added to hydrogen peroxide solution at a rate of 1:1 is heated at 80.degree. C. immediately before the respective processes, and then used, to thereby remove an organic substance, (hereinafter referred to as "wet ashing"). Immediately after then, a succeeding process is conducted. Conventionally, it has been thought that all of the organic substances can be removed by the dry ashing and the wet ashing. However, it has been found as a result of estimating the carbon contamination on the substrate surface through the known XPS measurement, that only C--C bonds (single bond of carbon) are hardly removed.
FIG. 2 shows a substrate surface 21 (indicated by a dotted line graph in FIG. 2) which has been subjected to a photo-resist coating, pre-baking, light-exposure, development, post-baking, and photo-resist peeling, and a surface 22 of the substrate (indicated by a solid line graph in FIG. 2) which has been further subjected to dry ashing and wet ashing, which have been measured using XPS. Under the measuring condition where the angle of a detector is set to 15.degree. in order to obtain the information of the surface as much as possible, an area 1 mm .PHI. on the substrate surface was measured. A horizontal axis represents a bonding energy with an unit of eV whereas a vertical axis represents the intensity of the detector with an arbitrary unit.
It is found from the graph of FIG. 2 that a peak in the vicinity of 284.8 eV is increased in both the cases of before (dotted line) and after (solid line) the substrate surface is subjected to dry ashing and wet ashing, and all of other peaks are decreased. The peak of 284.8 eV shows the existence of C--C single bond.
This shows that the removal of single bond of carbon is very difficult in the conventional dry ashing and wet ashing and almost impossible. Because carbon remains on the substrate surface as impurities, if, for example, an oxide film or the like is formed on the substrate surface, carbon remains on the interface between the oxide film and the substrate surface and forms a recombination center on the interface. Also, this develops charge capture and lowers the electric characteristic of a semiconductor such as the mobility of the thin-film transistor. Further, because the bonding state is not stabilized, an electric field continues to be applied, whereby the interface state is changed in time with a lost reliability.
Also, Japanese Patent Unexamined Publication No. Hei 4-177735 by the present applicant discloses that a bias is applied to a substrate using hydrogen of 100% to conduct plasma hydrogen cleaning on the semiconductor surface before forming a film by a sputtering unit. However, at the time of filing a Japanese Patent Application of the above Publication, because it has not been found that hydrogen radicals are effected on the single bond of carbon, a bias has been applied to the substrate to use the sputtering effect due to hydrogen ions, thereby cleaning the semiconductor surface.
For that reason, in order to make the interface characteristic excellent, because a balance of the effect obtained by removing the impurities and the damage caused by sputtering must be kept, the process margin cannot be increased so much. For that reason, processes which are capable of utilizing the plasma hydrogen cleaning have been limited.
Furthermore, not only the carbon contamination on the surface but also the carbon contamination in the film, using an organic silane source causes serious problems. A method of forming a film using normal tetraethyl silicate which has been well used is stated below. As the plasma CVD method, a substrate on which a film is formed is disposed within a chamber which has a parallel flat electrode and is capable of creating a vacuum in the chamber. In this situation, the one end of the parallel flat electrode is connected to the high-frequency source, connected to so-called cathode. The other one of the electrode is connected to the earth, and the substrate is disposed on the earth side electrode, that is, the anode side. Normal tetraethyl silicate is heated and increased in vapor pressure because it is liquid at room temperature, before being introduced into the chamber, or normal tetraethyl silicate bubbles with a carrier gas within a tank, and is then introduced together with the carrier gas into the chamber. Normal tetraethyl silicate resolved in plasma is characterized in that it forms a precursor and moves fluidly, thereby being capable of forming a film excellent in the step coverage property. The precursors that move on the substrate collide with each other, and also oxide ions, oxide radicals and ozones, and create an abstraction reaction to form SiOx. If a large amount of oxygen is introduced into the chamber, then the abstraction reaction from the precursor which is made of normal tetraethyl silicate on the surface facilitates, to form a film which is reduced in the amount of carbon but low in step coverage property.
When the amount of introduction of oxygen is slightly small, the step coverage property is improved, but the bond of carbon or oxygen and hydrogen exists much, thereby forming a film high in hygroscopicity. As a result of the measurement due to infrared absorption, the film is formed such that the absorption in the vicinity of 3660 cm.sup.-2 is increased much as a time elapses. This exhibits that the absorption in the vicinity of 3660 cm.sup.-2 is mainly caused by the Si--OH bond, and the formed film is hygroscopic.
As another method of forming a film using normal tetraethyl silicate, there is the atmospheric CVD method using ozone and heat. This is that normal tetraethyl silicate contained in a tank is bubbled with N.sub.2 on a substrate heated at 300 to 400.degree. C. and then heated in a reaction chamber, or oxygen is used for generating ozone through an ozonizer and then introducing it in the chamber. Since this method is high in step coverage property and also high in film forming rate, it is also used for an interlayer insulating film or the like, required for a multi-layer wiring such as LSI or DRAM memory. Thereafter, a so-called flatting operation is conducted in combination of etchback, SOG (SPIN ON GLASS), CMP (CHEMICAL MECHANICAL POLISHING), etc.
Among carbon contaminations on the substrate surface in a process of manufacturing the thin-film semiconductor device, particularly the impurities caused by a single bond (C--C) of carbon which can be hardly removed by the conventional wet ashing or dry ashing are reduced, thereby reducing the deterioration of the electric characteristic caused by the impurities of carbon on the boundary where a variety of semiconductors are formed, the lowering of the reliability, and so on. In particular, the carbon contamination on the interface between the active layer semiconductor and the gate insulating film is reduced. Also, in the case of forming a film with an organic gas such as normal tetraethyl silicate as a source, the hygroscopicity and the content of carbon are increased with an improvement in step coverage property, resulting in a lack of the reliability and the no-good property of the semiconductor characteristic. Moreover, a large amount of oxygen is added to the organic silane gas such as normal tetraethyl silicate in order to reduce the content of carbon, to thereby lower the step coverage property, disconnect a wiring, etc., resulting in a lack of the reliability and the no-good property of the semiconductor characteristic.